Olefin thermoplastic elastomer compositions composed of an olefin rubber and a crystalline olefin resin are excellent in flexibility and resistances to thermal aging and weather.
It is, however, unsuitable to use the conventionally known olefin thermoplastic elastomer compositions as high-functional materials. This is because most of the elastomer compositions are materials which are partially vulcanized by an organic peroxide, so that their oil resistance as well as their shape recoverability at high temperatures, such as compression set, are not satisfactorily high. Furthermore, the vulcanization using an organic peroxide brings about not only vulcanization of the olefin rubber but also crosslinking or cleavage of molecules of the crystalline olefin resin. For this reason, it is difficult to vulcanize the olefin rubber to a high degree, and the resulting elastomer composition is to have poor mechanical properties.
To improve the above shortcomings, a method in which a reactive alkylphenol-formaldehyde resin (hereinafter referred to as a phenol-type vulcanizing agent) is employed as a vulcanizing agent has been proposed as disclosed in Japanese Patent Publication No. 58-46138 and Japanese Laid-Open Patent Publication No. 59-91142.
According to the above method, only the double bonds in an ethylene-.alpha.-olefin-non-conjugated diene copolymer rubber are selectively reacted with the phenol-type vulcanizing agent, so that the state of vulcanization can be significantly improved. The copolymer rubber can thus be vulcanized to a high degree which cannot be attained when an organic peroxide is used as a vulcanizing agent, and the resulting elastomer composition has drastically improved oil resistance and shape recoverability at high temperatures.
Thanks to the above method, the olefin thermoplastic elastomer compositions became usable as substitutes for synthetic rubbers, such as chloroprene rubber, chlorosulfonated polyethylene rubber, ethylene-propylenediene copolymer rubber and acrylonitrile-butadiene copolymer rubber, which have been conventionally used in the fields requiring high functionality, such as parts of automobiles and industrial machines.
However, since the olefin thermoplastic elastomer compositions vulcanized by the phenol-type vulcanizing agent have low fluidity, they are poor in injection moldability. In particular, in the production of thin moldings there is a problem of poor filling in a mold.
Moreover, the injection moldings of the above elastomer compositions have high anisotropy in mechanical properties. The moldings therefore tend to have, depending on their shapes, a portion of remarkably inferior properties in terms of, e.g., tensile strength and elongation at break. Such moldings are not suited for practical use.